I don't want to get in the "less filling" "tastes great" battle about whether
multi grade oils thicken or stay the same over the 0 - 210f range but I do know
that 10-30 starts out holding about 40 PSI right after a change in my boat and
in about 15-20 hours it is more like 35. Something is going on.
That is using Merc oil or a good auto oil. If I bump it up to 25-40 it is
better. Straight 30 acts about the same as the 25-40.
Since I am never going to be in my boat if it is much below 70 degrees (air). I
am not sure why I am not just running 30HD. Cooling water will range from 67f
to 88f (5 year min/max) based on the water samples I do for the state.